In many chemical plants, a vessel needs to be temporarily closed, e.g. separated from an adjacent vessel, by an appropriate valve. In a closed position, the valve prevents solid, liquid and/or gaseous chemicals from entering and/or escaping from the vessel, while in an open position, the valve provides an opening for these chemicals. In many applications, e.g. in the petrochemical industry, the valve needs to withstand not only potentially aggressive chemical substances, but also high pressure and/or very high temperatures. Especially the latter condition presents a high challenge to the sealing properties of the valve.
One example for such extreme conditions is in the hydrocarbon processing industry, where valuable products are recovered from heavy residual oil by a process known as “delayed coking”. The residual oil is heated by a furnace and fed into a coke drum. In the process, heavy hydrocarbons are partially cracked to produce lighter substances that may be recovered from the vapors exiting the coke drum. On the other hand, an increasing amount of coke remains in the coke drum. When the coke drum is full, it is cooled and the coke is removed. According to a well-known design, the coke drum is connected to a gate valve on its upper side and its lower side. The removal of the coke is carried out by opening the upper and lower gate valve, also known as de-heading, and cutting the coke from the upper drum opening using a high-pressure jet of water.
During the coking process, the lower gate valve is subjected to extreme temperatures and/or severe pressure, which can lead to serious deformations and may cause leakage of hazardous gases. In particular, the part of the valve that faces the coke drum is subjected to high temperatures of several hundred degrees Celsius, while the parts facing away from the coke drum are subjected to relatively moderate conditions.
Among the gate valves used today for de-heading coke drums, two different designs can be distinguished: single gate valves and double gate valves. The single gate design, which is e.g. described in U.S. Pat. Nos. 6,660,131,6,565,714 or 6,964,727, employs a single gate that is movably disposed within a valve housing. The valve housing has a pair of opposite openings and a path between these openings is either closed by the gate or opened if a through-hole of the valve gate is placed in the path. This design is compact and light. However, the extreme temperature differences acting on the single gate lead to thermal deformation which makes it difficult to achieve a tight seal. At least one floating dynamic, live loaded seat is provided to act against at least one side of the gate. Such a floating dynamic, live loaded seat is continuously loaded against the gate to provide a biased relationship between the seat and the blind. The function of the dynamic, live loaded seat is also to provide point to point fine tuning of the system. The sealing seat is permanently pressed onto the gate and intense friction occurs when the gate is moved between the open and the closed position, which leads to increased abrasion and necessitates high drive forces. Further disadvantages are leakage and the need for an extensive amount of purge steam.
The double gate design, which is e.g. described in U.S. Pat. No. 5,116,022, employs two gates disposed parallel above each other, each of which engages a separate sealing seat in the closed position, so that a redundant seal is achieved. Also, each of the gates, which are spaced-apart and normally connected by intermediate spring elements, is subjected to rather moderate temperature gradients, which leads to reduced thermal deformation. Furthermore, the lower gate is not subjected to significant pressure. The gates are pressed onto their respective sealing seats by a wedge mechanism disposed between them and the pressing force is released when the gates are moved to the open position. Since there is no permanent pressing force, the above-mentioned friction problems can be avoided. However, this design is rather complex, heavy and bulky.